Method for permanent connection of two metal surfaces

ABSTRACT

A process for the production of a permanent, electrically conductive connection between a first metal surface of a first substrate and a second metal surface of a second substrate, wherein a permanent, electrically conductive connection is produced, at least primarily, by substitution diffusion between metal ions and/or metal atoms of the two metal surfaces.

FIELD OF THE INVENTION

The invention relates to a method for the production of a permanentconnection between two metal surfaces.

BACKGROUND OF THE INVENTION

The production of permanent, electrically conductive, metallicconnections between two metal surfaces is gaining increasing importancein the semiconductor industry. Primarily for new types of packagingtechnologies in the area of so-called “3D integrated devices or ICs (3DIC),” metallic bond connections between two functional planes play adecisive role. In this case, first active or passive circuits aremanufactured on two independent substrates, and the latter arepermanently connected to one another in a bonding step, and theelectrical contacts are established. This connection step can beaccomplished either by connecting two wafers (wafer to wafer—W2W), byconnecting one or more chips with a wafer (chip to wafer—C2W), or byconnecting one or more chips with a chip (chip to chip—C2C) method. Inthe case of these connection methods, direct connections between twoconnecting surfaces are of great interest, whereby both surfaces consistof the same material (metal) to a large extent. Here, methods that, to alarge extent, do not require additional materials in this connectionplane are quite especially preferred. In this connection, copper (Cu) oraluminum (Al) or gold (Au) is commonly used as metallization. It shouldbe clarified, however, that this invention basically also operates ininteraction with other metals, and the metal selection is basedpredominantly on requirements of chip structures and pre-conditioningsteps. Therefore, different metals are also to be regarded as claimedfor the invention. In addition, the method can also be used forso-called “hybrid bond interfaces.” These hybrid interfaces consist of asuitable combination of metal contact surfaces, which are surrounded bynon-metallic regions. In this case, the non-metallic regions areconfigured in such a way that in an individual connecting step, both themetallic contact as well as contacts between the non-metallic regionscan be produced. At this time, these connections, which are free offoreign materials, in particular of foreign metals, are produced by aso-called diffusion-bond method. Here, the contact surfaces are orientedtoward one another and are brought into contact. The contact surfacesare pretreated by means of suitable methods (for example, “ChemicalMechanical Polishing,” or, in short, “CMP”) in such a way that they arevery flat, and they have a slight surface roughness. The contactsurfaces are then pressed together in a suitable device (for example, awafer bonder) and are heated at the same time to a freely selectableprocess temperature. Here, it may also prove advantageous if this takesplace in an optimized atmosphere, such as, for example, a vacuum (e.g.,<1 mbar, preferably <1-3 mbar) or in a reducing atmosphere, inparticular an atmosphere with a high content (>1%, preferably >3%, evenbetter >5%, and ideally >9%) on hydrogen (H2). Under these processconditions, a so-called diffusion bond is now produced between the twometal surfaces. Here, in the case of eutectic metal compositions, metalatoms or molecules diffuse back and forth between the two surfaces andthus establish a permanent, metallically conducting and mechanicallyextremely stable connection between the surfaces. Often, in this case,the connection is of a quality that makes detection of the originalcontact surfaces in the metal structure impossible. Rather, theconnection is shown as a homogeneous metal structure, which now extendsbeyond the original contact surface. One factor, which greatly limitsthe use of this technology today, is the temperature that is relativelyhigh in most cases, which is necessary to produce the connection and inparticular to make the diffusion possible. In many cases, thistemperature is higher than 300° C., in many cases higher than 350° C.,typically 380 to 400° C., and in certain cases even up to 450 or 500° C.higher than the temperature that can be tolerated by the components(typically <260° C., in many cases <230° C., for certain components<200° C., and in certain cases <180 or even <150° C.) and thereforeprevents or limits the use of this method. This invention now deals withthis problem since it makes possible a method in which the necessaryprocess temperature is reduced dramatically.

These metallic connections are now to be referred to below in thisdocument as “authentic bond connections.” In this case, bond connectionsare always meant in which a connection is produced between two metalliccontact surfaces, consisting of a metal A, without the use of foreignmaterial installed permanently in the connection, in particular aforeign metal B, which has a different elementary composition.

As already described above, the methods that exist at this time arelimited by the necessary process temperature to make the diffusionprocess possible. In principle, it can be asserted that diffusionprocesses are actions that depend on multiple factors. However, it issuch that the process at lower temperature proceeds more slowly. Inpractice, however, this is a problem, since this would limit theeconomic efficiency of such processes, or would make very drawn-out (>1h) processes uneconomical. Therefore, the diffusion bonding processesare not applied between the same contact surfaces. As an alternative, inthis case, solder joints or the most widely varied manifestations ofeutectic connections and so-called intermetallic compound connectionapplications are used. As examples, solder joints based on lead/tinsolder, copper-silver-tin solder, indium-based solders or else gold-tinor gold-silicon or aluminum-germanium, as well as copper-tin(intermetallic compound Cu3Sn) can be cited here. The disadvantage ofthese methods lies in problems of both manufacturing logistics andtechnology. In many cases, these bond connections are to be produced inan area of manufacturing where only a certain metallization (e.g., Cu)is established and qualified. In this case, it would be an immenseadditional expense, in addition to this metallization, to build and alsoto qualify the infrastructure for another metallization. From thetechnological aspect, eutectic connections are to be considered criticalwith respect to the long-term stability. Certain connections areextremely brittle, and mechanical fatigue phenomena, i.a., can result.In addition, for certain metallizations, very narrow tolerances withrespect to the mixing ratio are observed to guarantee the desiredproperties (e.g., melt temperature, mechanical and electricalproperties) of the eutectic connection. In addition, diffusion effectsin connection with eutectic connections can cause problems. Thus, forexample, it would be a serious problem if tin were to diffuse from oneinterface between two copper contact surfaces through the entire coppercontact and were to reach the underlying barrier layer between thecopper contact and the underlying layer. Because of the altered metalcomposition, this would lead to the mechanical delamination of thecopper in this interface and thus result in a mechanical defect of thecomponent, which could occur only after several years in the field.These are effects that can occur in this form only with microstructures,since here, very thin layers are used in which such effects can onlyplay a role.

It is therefore the object of this invention to indicate a method withwhich a reduced process temperature and/or a reduced process time can beachieved in metallic bond connections.

This object is achieved with the features of claim 1.

Advantageous further developments of the invention are indicated in thesubclaims. Also, all combinations that consist of at least two of thefeatures that are indicated in the description, the claims and/or thefigures fall within the scope of the invention. In the indicated rangesof values, values that lie within the above-mentioned limits are alsodisclosed as boundary values and can be claimed in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a process for the production of apermanent, electrically conductive connection between a first metalsurface of a first substrate and a second metal surface of a secondsubstrate, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

This invention now presents methods and processes by means of which thebond temperature for authentic bond connections can be reduceddramatically, and this thus makes possible the use of connections, whichdo not require foreign metal, for a broad area of use. An additional useof this invention is also the acceleration of the process, which can beachieved in the case of ideal, selected process parameters and whichincreases the economic efficiency of the process.

Diffusion can be divided in general into substitutional and interstitialdiffusion.

In the substitution diffusion, the diffusion jumps of the individualatom take place along each point in the grid, on which other atoms couldbe located. As a result, such a diffusion jump can actually take place,if necessary, at the position to which an atom would jump but no otheratom can be located (exceptions exist: direct atom exchange mechanisms,which are discussed scientifically but are not yet detected; if theyexist, they occur so rarely in comparison to the other atom exchangemechanisms that they can be disregarded). The position at which no atomis located is called a void. The void is to occupy a fundamentallyimportant aspect in the further description of the patent.

In interstitial diffusion, smaller atoms diffuse within the latticeholes of a crystal. Since we are primarily dealing with homoatomicdiffusion in this patent, interstitial diffusion is not furtherconsidered.

Hydrogen, which diffuses in the lattice holes of the Si crystal grid,would be an example of interstitial diffusion. In comparison to Si,hydrogen is “small,” such that it has space in the lattice holes.

According to the invention, only the substitution diffusion is suitablefor the case of homoatomic diffusion of the same metal species.

In addition, surfaces, grain boundaries and volume diffusion can bedistinguished. Atoms diffuse the best where they are limited by as fewother atoms are possible. This state is primarily present on thesurface, by which the high mobility of the atoms on the surface is alsoclarified. Even in the grain boundaries, an atom in general has morespace than in the crystal grid itself. The speed of the diffusingspecies is therefore between that of the surfaces—and that of the volumediffusions. A requirement for a grain boundary diffusion is, of course,the existence of grain boundaries.

For this case of polycrystalline metal surfaces, the following problemsoccur with direct bonds.

First of all, polycrystalline materials consist of several grains thatare oriented differently relative to the surface to be bonded. Thisresults in that the surface consists of different crystallographicsurfaces. The different physical properties of the individual grainsurfaces have, in general, different oxidation, diffusion, adhesionproperties, etc.

Secondly, the grains have so-called grain boundaries, i.e., atom-freeareas in the angstrom to nanometer range, which separate the grains fromone another, in which atoms have a higher diffusivity than in the grainvolume.

Thirdly, the surfaces to be bonded in the rarest cases are free ofoxidation products.

The fact that polycrystalline surfaces are present in the worst casewith oxidation products and a non-zero surface roughness does not makeany direct welding possible. The surfaces do not lie completely flatupon contact but rather form pores in the interface. These “microscopicholes” are not confused with the above-described voids that are offundamental importance for the diffusion, while “microscopic holes” inthe interface prevent the jump of atoms “to the other side.”

In summary, it can be said that the modification of two surfaces isimplemented according to the invention in such a way that at the lowestpossible temperatures, the diffusion of atoms into one another has to beimplemented as simply as possible.

Diffusion can be promoted, for example, by the metal surfaces that areto be connected being configured in such a way that layers that are nearthe surface or, ideally, one layer that starts from the surface andreaches a certain depth “d” (layer thickness) in the material is madeavailable, and said layer has a structure that produces diffusion, inparticular primarily substitution diffusion, between the surfaces thatare to be connected. Below, methods are now described that make itpossible to produce this layer that is near the surface. Primarily, itcan prove advantageous to make available a layer that is near thesurface and that is less tightly packed. It is thus meant that the voidconcentration is correspondingly high. These surface defects now havethe advantage that in a temperature treatment, a reorganization of thestructure takes place that ultimately leads to tighter packing (and anelimination of the surface defects). When this temperature step nowtakes place, while the two metallic contact surfaces are in closecontact, the latter can be plastically deformed and thus also closeempty spaces in the interface; thus, it makes possible an even bettercontact and promotes the development of a diffusion bond between thesetwo surfaces. A series of variants is now provided for the conditioningof surfaces with which these layers can be produced:

Subsequent Production of Surface Defects:

With this method, the metallic contact surfaces are produced withmethods that are known from the prior art.

Common process steps in this respect are the deposition of a so-called“seed layer,” which is used to make possible an electrochemicaldeposition of metal (e.g., copper). In this case, the metallizationobtains the necessary structuring (in the contact regions andnon-metallic adjacent regions located around the contact regions) bymeans of lithography and the defining of a so-called plating mask. Afterthe electrochemical deposition of the metal, the latter is polished inmost cases by means of Chemical Mechanical Polishing (CMP) to ensureflat surfaces and a very low surface roughness (<2 nm, ideally <1 nm,even more preferably <0.5 nm root-mean-square [rms], measured by meansof 2×2 μm of AFM Scan). These methods are sufficiently known in theindustry. Depending on the configuration of the bond interface, thenon-metallic region that lies around the metal pads can consist ofsilicon dioxide or an organic insulation material or other suitablematerials. In this case, the topography between the metal regions andthe surrounding regions can either be selected in such a way that boththe metal regions and the non-metallic regions simultaneously come intocontact—so that no topography is present—or alternatively, thenon-metallic regions are slightly recessed compared to the metallicregions (e.g., approximately 100 A, preferably 1,000 A or 2,000 A) sothat only the metallic regions come into contact with one another.

As an alternative to the electrochemical deposition of the metal, othermethods such as sputtering or the like are also suitable.

Starting from a metallic contact surface that is produced with a surfacequality that corresponds to a large extent to the conventional diffusionbonding method, these methods that are now suitable are subjected toincorporating subsequent surface defects.

In one embodiment, these surface defects are produced by implanting gasions. More preferably in this respect, ions are selected that havesufficient mass to produce surface defects in the structure by“dislocation” of corresponding metal atoms or metal molecules. Gasesthat do not react with the metal, in particular noble gases such asargon, are to be considered as especially suitable in this respect. Forcertain applications, however, nitrogen or other gases with sufficientmass are also suitable. The decisive question here is the ratio of themass of the gas ion in comparison to the mass of the metal atom. Inprinciple, this implantation process can be achieved in any device thatallows the bombardment of the metallic surfaces with gas ions. This ispreferably found, however, with plasma-based systems. Preferred in thiscategory are so-called inductively coupled plasma systems (ICP) orso-called capacitively coupled plasma systems (CCP). With both systemsand in particular ICP systems, it is critical that the accelerationenergy for the ions be selected correctly to achieve the desiredproperties of the metal layer that is near the surface. With ICPsystems, this acceleration energy can be set by means of variable fieldstrength. With CCP systems, this acceleration energy can be optimized bymeans of a series of variants. According to the invention, for thispurpose, a self-bias voltage can be provided on the wafer receptacle,and, preferably, the latter can also be specifically set to influencethe acceleration energy of the ions. It is even more ideal, however, touse a so-called “dual frequency plasma” set-up. It is thus possible tocontrol the plasma density and temperature with one of the twofrequencies, while the acceleration energy can be influenced with thesecond (frequency applied to the wafer receptacle). The set-up operateseven more ideally when the frequency that is applied to the wafer isselected comparatively low (in comparison to the plasma systems with a13.56 MHz operating frequency that is common to the industry). Morepreferably, this frequency is less than 1 MHz; better results areachieved with frequencies <500 kHz, optimal results are achieved withfrequencies <200 kHz, and the best results are achieved with frequencies<50 kHz.

In a preferred embodiment, a stronger electrical field in a boundarylayer (sheath) is produced with an in particular additional DC voltage,by which ions are accelerated more strongly on the substrate surface.

Especially good results are achieved when the selected gas for theplasma production consists not only of the ions for production of thesurface defects, but also contains additional portions thatadvantageously influence the process. Here, for example, the addition ofhydrogen is especially suitable since hydrogen has a reducing action andthus prevents an oxidation of the metal surface, or even can remove analready existing oxide layer. In particular, hydrogen ions, which areimplanted in the metal surface, can have a permanentoxidation-preventing action that lasts for a few to several minutes(e.g., at least 1 minute, 3 minutes, or 5 to 10 minutes). Thus, anadequate time window is made available to be able, for example, toorient wafers to one another and to be able to apply the lattersubsequently for bonding in a bonding chamber. The implantation withvarious ions can in this case be carried out in parallel to achieve moreideal requirements by, as described above, a correspondingly selectedgas mixture being used, or else sequentially, by consecutive implantingsteps being performed with use of various process gases. This can takeplace either in the same or in different process chambers.

To date, subsequent to the production of surface defects, the contactingand bonding of the surfaces takes place as usual. Only the processparameters can be matched advantageously. In particular, the bonding cannow be carried out with significantly reduced process temperatures.Here, excellent results can already be achieved at temperatures <300° C.With optimized layers that are near the surface, a reduction of thetemperature to <260° C., ideally to <230°, in many cases to <200° C.,and in individual cases even to <180 or <160° C., is possible. As analternative, the process window can even be selected in such a way thatthe process time can be reduced in the case of a somewhat higher processtemperature.

Production of a Layer with Defects that is Near the Surface:

In the corresponding selection of the metal-depositing processes, it ispossible to produce metal layers of inferior quality. In most cases,this is undesirable, since the electrical conductivity of these layersis only limited. This can be attributed to a sub-optimal configurationof the metal structure. This effect is used according to the invention.In this case, first the metal surfaces are produced with methods thatare commonly used in the industry. Here, reference can be made to theembodiment above. Building on these layers, a very thin metal layer thatis of inferior quality is now applied. Typically, the thickness of thislayer is selected at <3 nm, better at <2 nm, and even more ideally,however, at <1 nm or <0.5 nm. This layer can be applied either to bothcontact surfaces or, as an alternative, even to only one of thesurfaces. In this case, the thicknesses are then correspondinglyoptimized. Then, the contact surfaces are brought into contact andheated as is common practice. In this case, the surface defects are noweliminated, and a diffusion bond is produced on the contact surfacebetween the two surfaces. The metal layer with the inferior quality inthis case promotes the development of this diffusion bond. Theproduction of this (these) layer(s) can be controlled by known processparameters on such deposition processes. The parameters that in thiscase influence the layer quality are known in the industry and can beseen in the pertinent literature. In most cases, here, these aredeposition temperature, ambient pressure in the process chamber of thedeposition system, as well as the selection of the gas and the ambientconditions, which is in the deposition chamber of the deposition system.A method that is suitable in this respect would be, for example, asputtering process, which is performed under process conditions that arenormally considered to be sub-optimal (such as, e.g., a processtemperature that is too low).

As an alternative, the layer can also be produced by means ofelectroplating. In this case, it is conceivable first to produce thesurfaces in planar form (as described above) and then to produce a thinlayer (layer thicknesses, see above) by means of electroplating. Basedon an optimized selection of the electroplating process (chemicalcomposition, current values, temperature, etc.), a layer can thus beproduced with the desired properties.

In terms of this invention, surface defects are of a size that in theideal case has a value of one or more atoms, in particular <10 nm,preferably <5 nm, even more preferably <3 nm, even more preferably <1nm, and even more preferably <0.5 nm, relative to a void in the shape ofa sphere or an equivalent shape.

Application of Metallic Nanoparticles:

It is already known from scientific literature that metal particles(e.g., gold, but also copper) with a size of <100 nm, ideally <70 nm,even better <50 to 30 nm, and preferably <20 or <10 nm, have theproperty of connecting to a homogeneous continuous metal structure in aheat treatment at temperatures below the melting point depending on thesize of the particles but also far below the melting point. Thisproperty can now be used for bonding processes by such particles beingapplied in the form of a thin layer on one or both metallic contactsurfaces. The contact surfaces are then brought into contact andsubjected to the heat treatment. During this heat treatment, thesenanoparticles make possible both the bonding below one another and thebonding with the metallic contact surfaces, and as an end result, acomplete bonding of the two metallic contact surfaces with one another.This is made possible in that the nanoparticles are very reactive ontheir own and have the property of ideally connecting with metalsurfaces, in particular consisting of the same metal. With use of smallparticles, this connection can be carried out even at temperatures<250°C., ideally <200° C., even more preferably <150° C., and even <120° C.in cases with very small particles.

Optimization of Surface Roughness

A similar acceleration of the diffusion bond and a bonding in particularat greatly reduced temperature is also another possibility tocorrespondingly optimize the surfaces with respect to surface roughness.The basic principle consists in the planarization of the surfacewaviness and micro-roughness. The root-mean-square (RMS) roughness is tolie within the nanometer range. The roughness that is set has to behomogeneous. This means that the mean wavelength, as well as the meanamplitude of the ridge-valley profile, which is measured with anatomic-force microscope (AFM), has to be the same on the entire surface.This is a necessary requirement in that the surfaces can engage over oneanother upon contact in such a way that the ridges of one surface fillthe valleys of the other surface and vice versa. Based on this optimalcontact, the development of a diffusion bond is greatly promoted and ismade possible even at lower temperatures.

The surface roughness that is necessary in this respect can be achievedby specifically selected CMP processes. On the one hand, CMP processesmake possible the production of a very flat surface, while the surfaceroughness can also be influenced with the suitable selection ofslurries. The production of the desired surface composition can in thiscase be carried out either in an individual CMP step or in two stepsthat follow one another. In this case, the first step is used to ensurethe planarity of the surfaces, while the second step is used to producethe desired local surface roughness. Optionally, surface roughness canalso be produced by means of a special etching step. In addition, it isconceivable to produce the required roughness with an interactionbetween electroplating and CMP or as a result of a specificallyperformed electroplating step. In this case, it is conceivable first toproduce the surfaces in planar form and then to produce a thin layer(layer thicknesses, see variant production of a layer with defects thatis near the surface) by means of electroplating. Based on an optimizedselection of the electroplating process (chemical composition, currentvalues, temperature, etc.), a layer with the desired properties can thusbe produced.

The surface roughness (measured with AFM for a 2×2 μm surface) should be<20 nm, in particular <10 nm, preferably <5 nm, even more preferably <3nm, even more preferably <1 nm, and even more preferably <0.5 nm.

To be able to achieve especially optimized process results, theabove-described variants can also be combined with one another asdesired. Primarily, an implanting of hydrogen as a measure to avoidoxidation in the interaction with the other described methods can yieldespecially optimized results.

It can be mentioned here one more time that the method also can beapplied to so-called “hybrid bond interfaces.” These hybrid interfacesconsist of a suitable combination of metallic contact surfaces, whichare surrounded by non-metallic regions. In this case, the non-metallicregions are configured in such a way that in an individual bonding step,both the metallic contact and contacts between the non-metallic regionscan be produced.

Here, it can be especially advantageous to configure the plasmaimplanting step in such a way that both the metallic connection at lowtemperature and the connection between the non-metallic areas thatadjoin the metallic regions can be produced. In this case, thesenon-metallic areas could consist of silicon dioxide, which also can bemodified by means of plasma treatment in such a way that the bonding cantake place at very low temperatures.

The invention consists in particular in a process flow 100, illustratedin FIG. 1, for the production of a permanent, electrically conductiveconnection between a first metal surface of a first substrate and asecond metal surface of a second substrate with the following methodsteps, in particular the course of the method:

Conditioning (step 101) of the first and second metal surfaces in such away that in a connection of the metal surfaces, in particular in a timeperiod of a few minutes after the conditioning, a permanent,electrically conductive connection—produced at least primarily bysubstitution diffusion between in particular similar, preferablyidentical metal ions and/or metal atoms of the two metal surfaces—can beproduced (step 103),

Orientation (step 102) and connection step (103) of the first and secondmetal surfaces, whereby during the conditioning, orientation andconnection, a process temperature of at most 300° C., in particular atmost 260° C., preferably 230° C., even more preferably 200° C.,especially preferably at most 180° C., and ideally at most 160° C., isnot exceeded.

The invention claimed is:
 1. A process for the production of apermanent, electrically conductive connection between a first metalsurface of a first substrate and a second metal surface of a secondsubstrate wherein said process does not include a soldering process,said process comprising the following steps: positioning the firstsubstrate and the second substrate such that the first metal surface ofthe first substrate and the second metal surface of the second substrateare spaced apart from each other, conditioning at least one of the firstmetal surface and the second metal surface by producing at least onelayer that is near at least one of the first metal surface and thesecond metal surface with voids formed therein, said voids produced byimplanting gas ions into at least one of the first metal surface and thesecond metal surface, after at least one of said first metal surface andsaid second metal surface are conditioned, orienting said firstsubstrate and said second substrate such that said first metal surfaceand said second metal surface are oriented toward one another, andconnecting the first metal surface and the second metal surface togetherthrough substitution diffusion between similar metal ions and/or metalatoms of the two metal surfaces, said substitution diffusion beingenabled by said producing of said voids, wherein, during theconditioning, orienting and connecting steps, a process temperature isless than 300° C.
 2. The process according to claim 1, wherein the firstmetal surface and/or the second metal surface has (have) a layerthickness S<5 nm.
 3. The process according to claims 1 or 2, whereinhydrogen is implanted to avoid oxidation.
 4. The process according toclaims 1 or 2, wherein the conditioning step comprises the optimizationof the surface roughness of at least one of the metal surfaces.
 5. Theprocess according to claim 3, wherein the conditioning step comprisesthe optimization of the surface roughness of at least one of the metalsurfaces.
 6. A method for the production of a permanent, electricallyconductive connection between a first metal surface of a first substrateand a second metal surface of a second substrate wherein said processdoes not include a soldering process, said method comprising the stepsof: (a) positioning the first substrate and the second substrate suchthat the first metal surface of the first substrate and the second metalsurface of the second substrate are spaced apart from each other, (b)conditioning at least one of the first metal surface and the secondmetal surface by producing voids that are disposed beneath at least oneof the first metal surface and the second metal surface, said voidsproduced by implanting gas ions into at least one of the first metalsurface and the second metal surface or by application of metallicnanoparticles to at least one of the first metal surface and the secondmetal surface, (c) after at least one of said first metal surface andsaid second metal surface are conditioned, orienting said firstsubstrate and said second substrate such that said first metal surfaceand said second metal surface are oriented toward each other, and (d)connecting the first metal surface and the second metal surface togetherthrough substitution diffusion between similar metal ions and/or metalatoms of the two metal surfaces, said substitution diffusion beingenabled by said producing of said voids, wherein, during theconditioning, orienting and connecting steps, a process temperature isless than 300° C.
 7. The method according to claim 6, wherein the firstmetal surface and/or the second metal surface has (have) a layerthickness S<5 nm.
 8. The method according to claims 6 or 7, whereinhydrogen is implanted to avoid oxidation.
 9. The method according toclaims 6 or 7, wherein the conditioning step comprises the optimizationof the surface roughness of at least one of the metal surfaces.
 10. Theprocess according to claim 1, wherein the conditioning step comprisesthe optimization of the surface roughness of at least one of the metalsurfaces to be less than 20 nm.
 11. The method according to claim 9,wherein the surface roughness of at least one of the metal surfaces isoptimized to be less than 20 nm.